Plastic shrinkage cracking in concrete usually occurs during the first 5 hours after placing and therefore the mechanics of fibre reinforcement were studied during this period. Two types of polypropylene fibres were mixed at 0.1% by volume. The development of bond strengths and the stresses which the fibres could sustain across cracks were measured by uniaxial tensile tests during the first 5 hours after mixing. Fibre stresses up to 130 MPa at 5 hours were achieved which were equivalent to a composite post-crack strength of 65 kPa. Restrained ring tests were used to assess the amount of cracking which occurred during the first 24 hours in a different high strength mix and, although the results were very variable, the fibres were found to reduce the crack area by between 40% and 85% compared with plain concrete, depending on fibre type

Tests were developed to quantify parameters affecting the plastic shrinkage cracking of high-strength concrete of 28-day cube strength in excess of 70 MPa. The parameters measured were tensile stress–strain performance during the first 5 h after mixing and negative pore pressure development and free shrinkage during the first 24 h. Eight high-strength mixes were used containing a variety of supplementary cementing materials such as microsilica, pulverised fuel ash, granulated slag and metakaolin. Two types of superplasticers were included. Plastic shrinkage cracking was assessed using restrained ring tests in which measurements were taken using sealed samples and samples exposed to wind. The research has shown that there is no simple relationship between early age stress–strain curves, negative pore pressure, early age shrinkage and macrocracking in adverse conditions but two factors were always present when plastic cracking was observed, these being microsilica and wind.

The development of equipment suitable for measuring tensile stress—strain curves on fresh concrete is described. With such weak materials, it is essential to eliminate friction effects and this is achieved by the use of air-bearing plates. The objective of the new equipment is to enable data to be obtained to define the rapidly changing tensile stress—strain performance in the first 6 h after water is added at the mixer. This information is important for understanding plastic shrinkage cracking. The equipment is also being used to investigate early-age internal cracking problems which have been observed with high-strength concrete and which are related to the internal tensile failure strain of the cement paste. Some typical tensile stress—strain curves obtained using the equipment are shown for two types of concrete.